The present invention relates to a bias-roller type image transfer technique in electrophotography. More particularly, the present invention is concerned with an image transfer member and an image transfer device of this type, a well as to an electrophotographic recording apparatus relying upon this type of technique.
In an electrophotographic recording apparatus such as a laser beam printer or a copying machine, a toner image is formed on a photosensitive member and is then transferred to a recording medium. A method called bias-roller transfer method is known as a method for transferring the toner image from the photosensitive member to the recording medium. This method is disclosed in Japanese Patent Examined Publication No. 52-33494 which corresponds to the U.S. Pat. No. 3,781,105.
The bias-roller, type transfer method employs a roller or like member, which presses a recording medium such as a sheet of paper, onto a photosensitive member while applying a bias voltage to generate an electric field by which a toner image is transferred to the recording medium. The pressing of the recording medium onto the photosensitive member may be conducted by a type of member other than a roller. For instance, the U.S. Pat. No. 3,691,993 proposes the use of a metal-coated brush as means for pressing the recording medium onto the photosensitive member. The bias-roller transfer method is superior in that it enables an image to be transferred at a high speed and with a high image quality.
The transfer roller used in this method has to be electrically conductive in order that it can apply the bias voltage, and is preferably elastic in order that it can adequately press the recording medium onto the photosensitive member.
To meet these demands, transfer rollers usually have a metallic core portion, an electrically conductive elastic layer formed on the metallic core portion and a resin layer formed on the conductive elastic layer and having a high electrical insulating power. A high voltage is applied to the conductive elastic layer through the metallic core. The conductive elastic layer is formed from a material containing a conductive additive such as carbon black and a plasticizer dispersed in a matrix of a rubber or the like material. This type of roller is shown in, for example, U.S. Pat Nos. 3,959,573 and 3,959,574.
Japanese Utility Model Unexamined Publication No. 53-129338 proposes a transfer roller which does not have such an integral conductive elastic member as that mentioned above. Namely, this transfer roller has an insulating elastic layer surrounded by a separate conductive layer which in turn is covered with an outermost resin layer having high electrical insulating power.
Proposed also is a transfer roller which is composed of a conductive elastic layer alone, without outermost layer of high insulating power. This type of roller is disclosed, for example, in Japanese Patent Unexamined Publication No. 53-77533 and also in the U.S. Pat. No. 2,807,233.
On the other hand, toner particles and carrier tend to attach to the surface of the transfer roller while the transfer roller is pressed onto the photosensitive member through the intermediary of the recording medium. A transfer device has been known which has a cleaning means, such as a brush, combined with a transfer roller. This type of transfer device is disclosed, for example, in the U.S. Pat. No. 3,847,119 and also in Japanese Patent Examined publication No. 52-33494 mentioned before.
When a transfer of a image is conducted by means of a roller of the type described above, a nip region for the recording medium is formed between the transfer roller and the surface of the photosensitive member. The area of the nip region varies according to the hardness of the transfer roller. In other words, a moderate range exists in the hardness of the transfer roller, i.e., in the stiffness of the elastic layer of the transfer roller. If a local portion of the transfer roller is held in contact with the photosensitive member, the transfer roller is deformed or convexed at such a portion, with the result that the quality of the transferred image is impaired and the life of the transfer roller is shortened.
In order to obviate this problem, image transfer devices have been proposed in which a transfer roller is mounted such that it can be brought into and out of contact with the photosensitive member, as disclosed in Japanese Utility Model Unexamined Publication No. 58-159556 which corresponds to the U.S. Pat. No. 3,907,421, as well as in Japanese Utility Model Unexamined Publication No. 63-4562. In the devices shown in this literature, the movement of the transfer roller is controlled in accordance with a signal from an operation detector which is incorporated in, for example, a paper feeding mechanism.